1. Field of the Invention
The present invention relates to an electronic device which is provided with a circuit for processing two quadrature AC signals by predetermined operations in response to an input signal thereby to output a vector sum of the operations and, more particularly, to an offset compensation device for compensating the offset which is caused in the vector sum due to the change in the characteristics and the deviation of analog circuits for the individual operations.
2. Description of the Related Art
The quadrature modulator for generating a modulated signal as the sum of products (or the vector sum) between two quadrature carrier signals and an input signal can realize various modulation schemes and constellations so that it is adapted for various communication devices or electronic devices.
In this modulated signal, moreover, there are superposed the components (as will be called the “offset”), which are caused to leak the carrier components or to deteriorate other performances by the differences of characteristics and the fluctuations of the circuit for multiply the two carrier signals individually in an analog area.
FIG. 21 is a block diagram showing an example of the configuration of a radio transmission device equipped with a quadrature modulator for compensating the offset.
To the two inputs of an offset compensator 51, as shown, there are inputted sending data, which are to be transmitted in parallel through an I-channel and a Q-channel individually corresponding to the aforementioned two carrier signals. Control information is inputted from the outside to the control input of the offset compensator 51. The output of this offset compensator 51 is connected with the input of a D/A converter 52 (for parallel D/A conversions individually corresponding to the aforementioned I-channel and Q-channel), the output of which is connected with the corresponding input of a quadrature modulator 53. The quadrature modulator 53 has its carrier input connected with the output of an oscillator 54 and its output connected with the feeding point of an antenna 56 through a power amplifier 55.
In the radio communication device (as will be called the “first prior art example”), the offset compensator 51 adds the aforementioned control information to either of signals (as will be called the “modulated signals” for simplicity) individually indicating the aforementioned two sending data. The D/A converter 52 converts the two modulated signals containing the modulated signal, to which the control information is added, into individual analog signals. The quadrature modulator 53 converts the carrier signals generated by the oscillator 54, into two quadrature carrier signals thereby to generate a modulated signal as the sum of products between those two carrier signals and the aforementioned analog signal. The power amplifier 55 sends the modulated signal at a desired level through the antenna 56.
Due to the difference in the characteristics of the D/A converter 52 individually corresponding to the I-channel and Q-channel, for example, of the components of the carrier signal to be fed by the oscillator 54 to the quadrature modulator 53, moreover, the aforementioned control voltage is manually set to the value, for which the component of the carrier signal observed at the output terminal of the quadrature modulator 53 (or the power amplifier 55).
Here, the offset due to the difference in the characteristics of the D/A converter 52 can be compensated, for example, by directly increasing/decreasing the DC reference voltage to be given to the D/A converter 52 (as will be called the “second prior art example”) or by directly increasing/decreasing the DC voltage to be superposed on one or both the aforementioned analog signals to be inputted to the quadrature modulator 53 (as will be called the “third prior art example”).
FIG. 22 is a block diagram showing another example of the configuration of the radio transmission device equipped with the quadrature modulator for compensating the offset.
The configuration of the radio transmission device shown in FIG. 22 (as will be called the “fourth prior art example”) is different in the following points from that of the radio transmission device shown in FIG. 21:
the output of the power amplifier 55 is connected through a directional coupler 57 to the feeding point of the antenna 56;
the monitor terminal of the directional coupler 57 is connected with the control input of the offset compensator 51 through a mixer 61, an A/D converter 62, a quadrature demodulator 63 and a feedback control unit 64, which are cascaded; and
the output of an oscillator 65 is connected with the local-frequency input of the mixer 61, and the output of an oscillator 66 is connected with the carrier input of the quadrature demodulator 63.
In response to the local-frequency signal generated by the oscillator 65, the mixer 61 frequency-converts the modulated signal given through the directional coupler 57, thereby to generate the monitored intermediate-frequency signal indicating the component of the modulated signal with an intermediate-frequency band.
The A/D converter 62 the monitored intermediate-frequency signal into a digital signal synchronized with a clock signal of a predetermined frequency.
In response to the “two quadrature carrier signals generated by the oscillator 66”, the quadrature demodulator 63 quadrature-demodulates the digital signal thereby to generate the quadrature-monitored signals i and q individually corresponding to the quadrature I-channel and Q-channel.
The feedback control unit 64 smoothes the quadrature-monitored signals i and q on a complex plane thereby to determine the offset components individually contained in those quadrature-monitored signals i and q, and feeds back those offset components to the offset compensator 51 thereby to compensate the offset, which is caused by the imbalance between the aforementioned I-channel and Q-channel, on the “section from the output terminal of the offset compensator 51 through the D/A converter 52 to the input terminal of the quadrature modulator 53”.
In this fourth prior art example, therefore, the quadrature demodulation is performed after the A/D conversion was made in the feedback passage. Unlike the later-described Patent Publication 4, for example, the feedback passage is not provided with the A/D converters individually corresponding to the aforementioned I-channel and Q-channel, and the compensation of the offset between those A/D converters is not required, so that the power consumption is reduced together with the cost and the size.
Here in the aforementioned first to third prior art examples, for example, the compensation of the offset is not stably performed, in case the difference in the characteristics of the D/A converter 52 corresponding to the I-channel and the Q-channel widely varies according to the environmental conditions and the aging. As a result, the components of the useless carrier signal may be contained in the modulated wave to be sent.
In these prior art examples, on the other hand, a spectrum analyzer or another dedicated device has to be applied for monitoring the components of the carrier signal, which are observed at the output terminal of the quadrature modulator 53 (or the power amplifier 55).
In the aforementioned fourth prior art example, on the other hand, the components, which are applied to the aforementioned A/D conversion by the A/D converter 62, of the clock signal can be transmitted to the output of that A/D converter 62 due to either the useless coupling in the A/D converter 62 between the different wirings or the characteristics of the elements. Moreover, the components of the clock signal thus transmitted are superposed on the “offset components determined by the feedback control unit 64 and adapted to be fed back to the offset compensator 51” especially in case they have a frequency equal to that of the “components of the carrier of the aforementioned monitored intermediate-frequency signal”.
In the fourth prior art example, therefore, the accuracy of compensations of the offset is not always high irrespective of the single number of the A/D converters to be mounted, and may highly vary according to the temperature or the aging.
[Patent Publication 1]
Japanese Unexamined Patent Application Publication No. 9-83587 (Abstract, FIG. 7)
[Patent Publication 2]
Japanese Unexamined Patent Application Publication No. 2000-270037 (Abstract, FIG. 1)
[Patent Publication 3]
Japanese Unexamined Patent Application Publication No. 2000-278345 (Abstract, FIG. 1)
[Patent Publication 4]
Japanese Unexamined Patent Application Publication No. 10-79693 (Abstract)